The Survival Value of Altruism and Selfishness

I've been reading a lot of entries and posts in both blogs and forums about the nature of good and evil and their relationship to systems of rules, such as the Ten Commandments.

I've a few rambling thoughts I'd like to share about that. The first has to do about the origin of good and evil. To start, I'd like to change that to altruism and selfishness. Granted, this is not a perfect translation, as it is much easier to label murder as an act of evil, than it would an act of extreme selfishness. Still, it works for the line of reasoning I'm about to pursue.

Why does humanity, in general, have some consensus in regards to what is good? Very simply, we evolved that way. Our apelike ancestors relied on numbers for defense. As such, it simply wouldn't do for members to go around stealing each other's food all the time, as that would result in some member starving to death or the extremely greedy ape being driven from the tribe (and likely dying). As such, high levels of selfishness would not have as much of a survival value, whereas behaviours that supported the continued existence of the tribe would. 

Of course, if any measure of selfishness were completely contrary to survival, we'd all have evolved to be saints. This is, clearly, not the case. It is quite likely that, in a number of circumstance, some degree of selfishness would have a higher survival value, such as when food was scarce. So this aspect of human psychology, too, would have carried on through our gene pool.  

Still, as we evolved, we developed more elaborate methods of interaction. With this, a greater emphasis on cooperative action led would have selected for more altruistic patterns of behaviour. 

Upon becoming human enough to develop culture, many people began to take on increasingly specialized roles, making cooperation between members absolutely vital. Again, this placed greater survival value on altruistic behaviour. 

As such, in general, human beings towards altruistic behaviour, at least towards their own culture groups. 

Are there problems with this line of thinking? Oh, quite probably, but bringing them up and talking about them will be half the fun of this thread.